[#136]Ability to tune what to write to admin log

-- Add the ability to enable or disable the admin log in the
configuration > my store

In v2.0, we will add a error log module to record the errors 
such as fatal Error, warning, Parsing Error and Notice error 
etc.

Store owner could set the Display Errors, Log Errors, Errors 
Level and Error Log Filename in the configuration. 

Moreover, he could check the errors log in the error 
log module.